Canola is an important agricultural crop in Canada, the United States, Europe and Australia. Weed competition and earliness of maturity are significant limiting factors in canola crop production and quality. The challenge for plant scientists has been to develop canola varieties having superior performance with respect to these limiting factors, while at the same time having satisfactory agronomic characteristics, including yield potential, lodging resistance, oil and protein content, and glucosinolate levels that are sufficiently low for registration.
Resistance to AHAS-Inhibitor Herbicides
Herbicide resistant plants are plants that are able to survive and reproduce following exposure to herbicides at rates of application that would prevent non-herbicide resistant varieties of the same species from surviving and reproducing. Herbicide resistance is particularly important for Brassica, since many weeds, such as stinkweed, shepherd's purse, flixweed, ball mustard, wormseed mustard, hare's ear mustard and common peppergrass have a close genetic relationship with Brassica. Therefore, it is advantageous for a cultivar to have herbicide resistance not possessed by related weeds.
Some herbicides function by disrupting amino acid biosynthesis in affected species. For example, AHAS-inhibitor herbicides (also known as ALS-inhibitor herbicides) function by inhibiting the enzyme acetohydroxy acid synthase (AHAS), the first enzyme in the biosynthesis of the amino acids, isoleucine, leucine and valine. In plants with resistance to an AHAS-inhibitor herbicide, inhibition of the AHAS enzyme is prevented, thus allowing the plant to continue with normal amino acid biosynthesis. Most forms of Brassica are highly susceptible to AHAS-inhibitor herbicides, such as imidazolinones and sulfonylureas.
The development of canola with resistance to imidazolinones, such as PURSUIT™ and ODYSSEY™, was a major breakthrough in weed management technology. The imidazolinones are a family of broad spectrum herbicides which may be applied for in-crop weed control. They control a larger number of problem species than herbicides used in non-herbicide resistant varieties, and offer greater management flexibility, including timing of application and tank mixing. An advantage of imidazolinone (“IMI”) resistant varieties over many other herbicide resistant varieties, such as ROUNDUP READY™ (glyphosate) or LIBERTY LINK™ (glufosinate) resistant varieties, is that some imidazolinone herbicides have a soil residual which controls successive weed flushes. This provides a significant advantage to farmers, because it enables them to achieve longer term weed control without a second application of herbicide. Effective weed control increases yield by reducing competition from weed species. It also improves grain quality through the elimination of cruciferous weed seeds. It may also improve weed management in other crops in the rotation, due to reduced weed pressure.
However, a drawback of currently available IMI resistant varieties is that they lack many of the desirable traits found in elite varieties of non-herbicide resistant canola. In particular, none of the currently available canola varieties have the desirable combination of IMI resistance and early maturity (Early Napus). It is particularly difficult to develop varieties having IMI resistance, in combination with other desirable traits, because the inheritance of the IMI resistance trait is relatively complex. Unlike the ROUNDUP READY™ trait or LIBERTY LINK™ trait, which are controlled by single transgenes that exhibit complete dominance, the IMI resistance trait is controlled by two unlinked gene pairs having partial dominance. Swanson et al., Plant Cell Reports 7:83-87 (1989) reported the development of imidazolinone herbicide tolerant Brassica napus mutants using microspore mutagenesis. During the process, five fertile double-haploid Brassica napus mutant plants were developed. One of the mutants was tolerant to between 5 and 10 times the recommended field traits of an imidazolinone herbicide. An inheritance study indicated that two semi-dominant unlinked genes combined to produce an F1 with greater tolerance than either of the parents.
Rutledge et al., Mol. Gen. Genet. 229:31-40 (1991) proposed a model for the inheritance of the five AHAS genes in Brassica napus. AHAS2, AHAS3 and AHAS4 appear to be associated with the ‘A’ (rapa) genome and AHAS1 and AHAS5 are likely associated with the ‘C’ (oleracea) genome. AHAS1 and AHAS3 are expressed at all growth stages (Ouellet et al., Plant J. 2:321-330 1992) and mutant forms of AHAS1 and AHAS3 appear to be the most effective tolerance genes. AHAS2 was found to be active only in ovules and seeds. AHAS4 was found to be defective due to interrupted sequences in the middle of the coding region (Rutledge et al., Mol. Gen Genet. 229:31-40 1991) and was not expressed in tissues examined by Ouellet et al., Plant J. 2:321-330 (1992). The last gene AHAS5, may also be defective (Rutledge et al. Mol. Gen Genet. 229:31-40,1991). Hattori et al. Can J. Bot: 70:1957-1963, (1992) determined that the DNA sequence of the coding regions for AHAS1 and AHAS3 were 98% identical. DNA sequences of the 5′ and the 3′ ends were also closely related. Few similarities were observed between the sequence of the AHAS2 compared to the AHAS1 or AHAS3 genes.
There are two effective mutations for IMI resistance in commercial use—an AHAS1 mutant (believed to be located on the C genome) and an AHAS3 mutant (believed to be located on the A genome). The AHAS3 mutant also provides resistance to other AHAS-inhibitor herbicides, such as sulfonylureas. The complexity of the inheritance of the IMI resistance trait results in multiple phenotypes during segregating generations, which presents a significant hurdle to plant breeders. Accordingly, there is a need to develop an AHAS-inhibitor herbicide resistant canola variety with improved performance characteristics.
Early Napus
Early maturity is an important trait in Brassica napus varieties, especially in market areas with a limited frost-free period. Late summer frosts can damage the crop before it is fully mature, resulting in elevated green seed content of the grain (a grading criterion) and increased chlorophyll in the oil (a quality problem). High green seed results in losses to the producer, while elevated chlorophyll in the oil increases processing costs, and results in a loss of value for food end users. Early Napus is also important where early maturity reduces exposure to extreme heat and drought conditions during flowering and seed-filling.
To be classified as “Early Napus”, a variety must have an average maturity which is at least four days earlier than the average maturity of the current WCC/RRC (Western Canadian Canola/Rapeseed Recommending Committee) check varieties (DEFENDER™, EXCEL™, and LEGACY™) over two years at 11 locations in the Short Season Zone of Western Canada. No known varieties of Brassica napus have the desirable combination of Early Napus and resistance to an AHAS-inhibitor herbicide, such as an imidazolinone. Therefore, there is a need for a Brassica napus variety which combines the advantageous traits of early maturity (Early Napus) and resistance to AHAS-inhibitor herbicides.
Accordingly, it is an object of the present invention to provide an improved variety of Brassica napus having early maturity (Early Napus) and resistance to at least one AHAS-inhibitor herbicide, such as an imidazolinone. These and other objects of the invention will be apparent to those skilled in the art from the following description and claims.